1. Field of the Invention
The present invention relates to an electrostatic powder spray gun for applying a charge to powder particles and blowing the same onto an object to be coated, utilizing electrostatic electricity.
2. Description of the Related Art
From the viewpoint of environmental conservation, an electrostatic powder spray gun attracts attention as an environment-friendly, non-polluting coating method without the need of using a solvent. In electrostatic powder coating, powder particles are supplied from a powder hopper to a spray gun via an injector so that the powder particles are sprayed with a conveying air flow to an object to be coated from a nozzle opening formed at the tip portion of the spray gun. At such time, the object to be coated is grounded, and a high voltage is applied to a pin type charging electrode(s) provided at the tip portion of the spray gun so that a corona discharge is generated from the charging electrode to the object to be coated. Therefore, the powder particles discharged from the nozzle opening collide with ions generated by the corona discharge so as to be charged during passage of the discharged particles in the vicinity of the electrode. The powder particles accordingly charged are coated on the surface of the object to be coated by the conveying air flow and along lines of electric force.
However, it is known that, in general, in powder coating, charged powder particles discharged from the tip of the spray gun can partially adhere to the periphery of the spray gun without contributing to coating film formation. If the powder particles continue to partially adhere to the spray gun, the adhered powder particles gradually aggregate on the periphery of the spray gun to generate the risk of a so-called spit, where the aggregated particles are flipped onto the object to be coated and adheres to the coated surface. Besides, since part of the powder particles do not contribute to film formation, a problem is involved in that the transfer efficiency of the powder particles is deteriorated.
Further, with a smaller size powder particle, the powder particle is lighter, so that the ratio of particle adherence to the periphery of the spray gun increases and the transfer efficiency further deteriorates.
Moreover, by providing an electrically-grounded ion trap electrode(s) rearwardly with respect to the charging electrode of the spray gun, for trapping free ions generated by the corona discharge, since an electric field is formed between the charging electrode and the ion trap electrode, part of the powder particles are drawn to the rear side of the spray gun by the electrostatic attracting force from the electric field so as to be easily adhered to the outer peripheral portion of the spray gun.